The present invention relates to Josephson junctions.
A Josephson junction has a barrier extending from one superconductive electrode to another superconductive electrode. Under certain conditions, electrons in one of the electrodes can tunnel through the barrier to the other electrode, in accordance with the Josephson effect.
The present invention provides a novel barrier for making Josephson junctions. The Josephson junction comprises first and second electrodes, each of which is formed of superconductive material. The first electrode has a first electrode face. A barrier of the junction extends from the first electrode to the second electrode. The barrier has a first barrier face opposing and adjoining the first electrode face. The barrier is formed of non-superconductive barrier material and superconductive barrier material. A concentration of the superconductive barrier material is greater than zero at the first barrier face, whereby the first barrier face is formed at least partially of the superconductive barrier material.
In a preferred embodiment of the invention, the concentration of the superconductive barrier material is 100% at the first barrier face, whereby the first barrier face is formed entirely of the superconductive barrier material. The junction has a plurality of distinct contiguous portions extending sequentially from the first electrode to the second electrode. The concentration of the superconductive barrier material is uniform along one of the portions, and the portion is preferably free of the superconductive barrier material. The first and second electrodes are formed of the superconductive barrier material. The concentration of the superconductive barrier material declines away linearly from the first barrier face in a direction toward the second electrode. The superconductive barrier material has a chemical formula. The chemical formula includes Yttrium, and the declining away of the concentration is characterized by the spatially graduated replacement of the Yttrium in the chemical formula by Praseodymium.
Preferably, the second electrode has a second electrode face, and the barrier further has a second barrier face opposing and adjoining the second electrode face. The concentration of the superconductive barrier material is greater than zero at both barrier faces, whereby both barrier faces are formed at least partially of the superconductive barrier material. Preferably, the concentration of the superconductive barrier material is 100% at both barrier faces, whereby both barrier faces are formed entirely of the superconductive barrier material. The concentration of the superconductive barrier material declines away from the first barrier face in a direction toward the second barrier face and declines away from the second barrier face in a direction toward the first barrier face. The concentration of the superconductive barrier material varies symmetrically about a midpoint between the barrier faces.
The invention also provides a Josephson junction comprising non-superconductive material and superconductive material. The superconductive material is mixed with the non-superconductive material in a spatially varied concentration to yield a spatially varied critical temperature of the junction. The spatially varied critical temperature has minimum and maximum critical temperatures. When the junction is cooled to a temperature between the minimum and maximum critical temperatures, the junction will have two superconducting zones extending from opposite ends of a non-superconducting zone. The length of the superconducting zone and the lengths of the non-superconducting zones in total equals the total length of the junction. The junction will be responsive to a decrease in temperature by effecting an increase in lengths of said superconducting zones and a corresponding decrease in length of said non-superconducting zone.
The invention further provides an apparatus for a sputtering system having a composite target made of two materials. The sputtering system includes a radiation source and a substrate. The apparatus comprises a target body having a peripheral surface. A first portion of the peripheral surface is formed of a first material to be sputtered from the target and deposited onto the substrate in a first concentration. A second portion of the peripheral surface is formed of a second material to be sputtered from the target and deposited onto the substrate in a second concentration. The target body is configured to enable variation of the first concentration relative to the second concentration by variation of the orientation of the target body relative to the radiation source to thereby vary the areas of the respective surface portions facing the radiation source.
In a preferred embodiment, the variation of the orientation of the target body is through rotation of the target body. The target body is cylindrical, and the first and second portions are disposed on radially-opposite sides of the cylindrical target.